[10.07] Could Asteroid 1997 XF11 Collide with Earth?

P. W. Chodas, D. K. Yeomans (JPL/Caltech)

Asteroid 1997 XF11 received much notoriety in March 1998
when for a time, orbit solutions indicated that it would
make a remarkably close approach to the Earth on October 26,
2028. The miss distance calculated from these orbit
solutions was less than one quarter of a lunar distance, and
possibly even smaller, making it easily the closest-ever
predicted approach of a minor planet to the Earth. The
fairly large size of the asteroid, probably over a kilometer
across, also made the object notable. Interest in this
object spread rapidly when initial reports to the press
suggested that a collision in 2028 could not be ruled out.
But, in fact, a complete analysis of the observations
available on March 11 (an 88-day data arc) shows that the
probability of impact in 2028 was very tiny, essentially
zero. When XF11's position uncertainty ellipse is plotted in
the plane perpendicular to the geocentric velocity vector,
the ellipse is seen to be extremely elongated, over 1000
times longer than its width. This elongation is due to the
fact that the position uncertainty along the orbit grows
linearly with time over the 30-year prediction period, while
uncertainties perpendicular to the orbit vary only
periodically. The great length of the uncertainty ellipse
makes it difficult to predict a precise miss distance, but
the narrow width of the ellipse allows the computation of a
likely minimum possible miss distance, about 28,000 km in
the case of XF11's passage in 2028. As it turns out, on
March 12, additional pre-discovery images of the asteroid
were found, which greatly strengthened the orbital solution
and moved the predicted close approach out to an
unremarkable 980,000 km. But these observations were not
needed to rule out the possibility of a collision in 2028.
We also investigate the close Earth approaches of 1997 XF11
and associated uncertainties for a few decades beyond the
year 2028.